Connected Spiral Antennas for Wideband Circularly Polarized ...

Author manuscript, published in "The 7th European Conference on Antennas and Propagation, GOTHENBURG : Sweden (2013)"
Connected Spiral Antennas for Wideband Circularly
Polarized Antenna Array, Experimental Investigations
Mohammed Serhir 1 , Regis Guivarc’h 2
1 Département de Recherche en Électromagnétisme, Supelec, Gif-sur-Yvette 91192, France (mohammed.serhir@supelec.fr)
2 Laboratory SONDRA, Supelec, 91192 Gif-sur-Yvette, France (regis.guinvarch@supelec.fr).
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Abstract—A cavity backed array of 5 connected spiral
antennas is presented. The circular polarization is generated
using mono polarized spirals in an alternating configuration
RHCP and LHCP. Due to the connection between spirals, the
currents in the arms of one spiral flow into the arms of the
adjacent spirals. The currents transmitted to the opposite
polarized neighboring spiral radiate the same polarization. The
proposed antenna array allows more than 500MHz bandwidth
while being dual polarized and steerable +/- 30deg. The
performance of the constructed array is presented and validated
using electromagnetic simulations and radiation pattern
measurements.
Index Terms— antenna array, circular polarization
I. INTRODUCTION
RADAR applications and synthetic aperture radar mapping
require wideband phased arrays. For these applications, spiral
antenna is an interesting broadband structure since its working
principle is governed by simple rules.
In fact, the spiral antenna bandwidth minimum frequency
(VSWR

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Figure 2. The elementary spiral antenna (left) and its feeding system (right)
Figure 3. Calculated reflection coefficient of the elementary spiral
The elementary spiral antenna feeding system is made of
two RG-10 coaxial cables soldered together where the inner
conductor of each cable is connected to the spiral arms. Doing
this, the impedance between inner conductors (seen by the
antenna) is also about 100Ω. On the other coaxial cables end a
180° hybrid coupler is used to generate two opposed-phase
signals. This versatile excitation technique can be generalized
to any symmetrical structure for which the input impedance is
around 100Ω. Using the electromagnetic simulation software
CST Microwave Studio [4] we calculate the spiral antenna
reflection coefficient as a function of the frequency when it is
excited by the feeding system described above.
As it can be seen from Fig. 3, the antenna S11

and simulated far-field radiation pattern resulting from the
prototype presented in Fig. 7. As it can be seen a good
behavior agreement is noticed. Based on measurement data,
the spiral array has a 650MHz bandwidth from 0.85GHz to
1.5GHz. This bandwidth is confirmed by a good S11 (below-
10dB).
in Fig. 9, the use of the lumped resistive loads does not affect
drastically the radiation pattern.
without resistive loads
Figure 5. Measured S11 coefficients as a function of the frequency
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Figure 6. Comparison of the simulation and measurement Axial Ratio as a
function of the frequency in the broadside direction (θ=0°).
resistive loads
Figure 7. The antenna array with lumped resistive loads.
with resistive loads
without resistive loads
Figure 8. Axial Ratio issued from simulation and measurement data as a
function of the frequency in θ=0° direction for spirals with resistive loads.
In Fig. 9, the measured electric field over a hemisphere (-
90°≤θ≤90° and 0≤φ≤180) is presented for frequencies inside
the antenna array bandwidth ([0.85GHz 1.35GHz]). As shown,
with resistive loads
Figure 9. Radiation pattern of the spiral array in two different frequencies
0.86GHz and 1.34GHz.

IV.
CONCLUSION
A versatile and efficient cavity backed circularly polarized
array of spiral antenna has been presented. This spiral antenna
array guarantees 500MHz bandwidth where the VSWR is less
than 2 and the axial ration less than 3dB. In this paper we have
excited port2 and port4 to reach the right hand circularly
polarized array (RH). To achieve the left hand circular
polarization (LH) port 1, port 3 and port 5 have to be excited.
Consequently this antenna array structure allows us to realize
LHCP and RHCP polarization. The minimum frequency of the
array is less than the minimum frequency of a single spiral
antenna composing the array structure.
REFERENCES
[1] R. G. Corzine and J. A. Mosko, Four-Arm Spiral Antennas. Boston,
MA: Artech House, 1990.
[2] R. Guinvarc'h and R.L. Haupt, “Connecting spirals for wideband dual
polarization phased array,” Antennas and Propagation, IEEE
Transactions on, vol.59, no.12, pp.4534-4541, Dec. 2011
[3] M. McFadden, W. R. Scott Jr.,” Analysis of the Equiangular Spiral
Antenna on a Dielectric Substrate” Antennas and Propagation, IEEE
Transactions on, vol.55, no.11, pp.3163-3171, Nov. 2007
[4] CST Microwave Studio, Tutorial
hal-00834795, version 1 - 17 Jun 2013